This invention relates in general to a hybrid lithographic process, and more particularly to a hybrid lithographic process for producing acoustic wave devices.
In the semiconductor industry optical lithography techniques are widely used for patterning semiconductor substrates or for patterning thin layers of material overlying semiconductor substrates, PC boards, and the like. In the conventional optical lithography process a layer of photoresist is applied and portions of the photoresist material are exposed, usually through a mask, to cause exposed portions to have different dissolution rates in a photoresist developer, one of the exposed or unexposed portions is removed to leave a patterned mask layer on the underlying material. The patterned photoresist layer then may be used as an etch mask, ion implant mask, metal lift-off mask, or the like.
It has been conventional for a number of years to optically expose the photoresist layer through a mask. Exposure is accomplished with actinic radiation, usually having an intensity peak in the ultra violet spectral region. As the semiconductor technology progresses, there arises a need for producing patterns having very small size or critical dimension. As the size of the photoresist patterns decreases to a range of less than one micrometer it becomes impossible to resolve such small sizes with optical techniques.
In view of the limitations on optical techniques, some work has been done on the use of electron beam exposure of the photoresist layer. Electron beam exposure is capable of delineating fine pattern geometries, but has the disadvantage that exposure of large areas is very time consuming because the exposure is made with an electron beam of small cross sectional area. Because of the nature of resist materials, an all electron beam exposure process would, in many cases, require the time consuming exposure of large areas.
Resist materials can be classified as either negative or positive resist. With negative resist, the unexposed resist portion is removed during the developing process; with positive resists, the opposite is true and only the unexposed portions remain after developing. In developing a pattern, such as a pattern on a complex acoustic wave device, in which fine geometries are required but in which only a small proportion of the total surface area is to remain covered, the use of negative resists and an electron beam exposure process are advantageous. The electron beam provides the necessary high resolution pattern but only a small portion of the resist layer would have to be exposed. To use a positive resist in this application would require the time consuming exposure of a large proportion of the substrate area.
To achieve the high resolution obtainable with electron beam resist exposure and yet maintain a production capable process having high throughput, it is desirable to combine electron beam and optical lithography in a single hybrid process. Others have disclosed the use of hybrid technology, but prior art process have not been entirely satisfactory.
In one disclosed process using a single layer of resist, a positive resist has been used; but in the electron beam exposure portion of the process the resist has been reversed from positive to negative. Optical processing then proceeds in a normal manner as with any positive resist. This process has the disadvantage of poor resolution, decreased resist contrast, and the need for higher, and thus longer, electron beam does.
In another disclosed process only a single layer of positive resist is used. In this process the area immediatly surrounding the fine lines is exposed with an electron beam and the remaining area to be removed is exposed with an optical process. The two exposed areas are then removed. This process has the disadvantage of being difficult to regulate to insure the proper areas are exposed.